Plastic powders develop directly during polymerisation of corresponding monomers (for example in suspension polymerisation), by mechanical comminution in a mill appropriate for the respective plastic and for the required degree of comminution, by spraying a solution or melt of the plastic by means of an appropriate nozzle through which the solution/melt is pressed under high pressure, and by dissolving of the plastic in an appropriate solvent and precipitation by reduction of the temperature and/or increase of the polymer concentration in the solution.
As applications of such precipitated powders made of polyamide 11 and polyamide 12 the literature (handbook of plastics “Polyamide”, Carl Hanser Verlag, Munich Vienna, 1998, chapter 4.14, pages 746-756) mentions: coating of metals by dip coating in a fluidised bed; electrostatic powder coating; and addition to coil-coating varnishes. Further, the use of polyamide powders in cosmetics is known (described in the firm brochure concerning Orgasol of the firm Atofina; obtainable in the internet at www.atofina.com with the keyword Orgasol Cosmetics).
For a person skilled in the art, it is perspicuous that a high bulk density and a high pourability are important for the mentioned applications of these powders. Moreover, they shall have a small BET-surface measured according to DIN ISO 9277. These characteristics depend on the shape of grains. Therefore, those shapes of grains are favoured which are free of sharp edges (spherical) and which are not porous. This demand is also and particularly valid for laser sintering for which the advantages of the powder according to the invention will be described in detail.
The demand is only fulfilled by such plastic powders formed directly during polymerisation. Powders formed by milling or precipitation according to the prior art do not fulfil the demand.
Milled powders additionally comprise the disadvantage of a broad distribution of grains. As a result, since the applications require limited bands of grains, they must comprise a subsequent classifying process in which part of the powder has to be removed as a worthless by-product.
Precipitated powders, sprayed powders or powders attained from melt dispersion often have grain size distributions necessary for the envisaged application. For example, EP 863 174 discloses a method for producing polyamide powders for coatings wherein a homo-polyamide or a co-polyamide is precipitated from an alcoholic solution. A polyamide powder produced by this method has an upper grain size limit of 100 μm, a D0.9-value below 90 μm, a D0.1-value below 32 μm and a BET-surface below 10. However, the porosity expressed as BET-surface is still high. Thereby, the process of precipitation arrives at its limits, since with constant or decreasing mean grain, only a defined range for adjusting the BET exists.
In laser sintering of plastic powders, as is known from DE 44 10 046, for example, three-dimensional objects are manufactured in a layerwise manner by applying layers of a powder and bonding them to each other by selective solidification at positions corresponding to cross-sections of the objects. According to the method characteristic, specific demands are put onto the powder.
To provide for a high precision in detail and quality of surface of the objects to be produced, plastic powders are required to have an upper grain size limit of 100 μm and a 90%-fraction (D0.9-value) below 90 μm. Furthermore, to provide for a stable application of layers, the powders must have a 10%-fraction (D0.1-value) of less than 32 μm. Furthermore, the particles should have a spherical grain shape. This grain shape is also necessary to provide for a smooth and even surface.
When using plastic powders for laser sintering, a low porosity of the particles, expressed as BET-surface, is required, since thereby the density of the powder bed can be increased and the reaction tendency and aging of the powder are strongly decreased. The last mentioned characteristics are very important in laser sintering, since high temperatures occur during the process and the process time may be very long which, depending on the type of plastic, may lead to build-up and decomposition processes and thus may complicate the recyclability of the powder. The latter shows in a high refreshing factor, which, in recycling of powder, is given by the percentage of fresh powder that has to be added to the recycled powder to avoid a variation in the laser sintering and object characteristics compared to a laser sintering process using fresh powder only.
By the large BET-surface, ageing of the unexposed powder during the sintering process is promoted whereby a high refreshing factor becomes necessary to prevent occurrence of drawbacks in surface quality and mechanical properties of the sintered objects.
Therefore, it is an object of the invention to provide a powder optimised with respect to pourability and porosity which is particularly suitable for laser sintering, and to provide a laser sintering method using this powder as a build-up material.
The object is attained by a powder according to claim 1 and by a method according to claim 12.
Further developments of the invention are specified in the subordinate claims, respectively.
Further features and advantages of the invention will arise from the description of embodiments with reference to the figures, of which: